Vibration damping devices for vehicles, such as engine mounts, transmission mounts, bushings, chassis/suspension insulators, and the like are used in motor vehicles for many reasons, among these reasons are insulating drivers and passengers from engine vibrations, engine noise, and road noise. Recent general trends in motors vehicles, particularly automobiles, have been that the size of vehicles is getting smaller compared to the automobiles generally available during the first three quarters of the 20th century. Additionally, the vehicles are becoming more aerodynamically designed. These two factors, among others, generally make the engine compartments of today's motor vehicles smaller than engine compartments of earlier automobiles. Even with the decreasing size of engine compartments, more functions and equipment are finding their way into the engine compartment. Additionally, today's smaller engines with higher revolutions and more torque are now powering vehicles.
The combination of these factors lead to higher and higher temperatures in the engine compartment or under the hood of a motor vehicle. Such increasing temperatures put additional stresses on parts in the engine compartment. As an example, in Northern latitudes, extreme low ambient temperatures will be experienced by majority of the components in the automobile. At those low temperatures, the rubber parts must retain much of their original flexibility to insure correct function. Upon starting and after warm-up, the engine compartment temperature, will be substantially the same in most latitudes. Accordingly, the low temperature performance specification for most automobile parts is generally fixed by the most extreme ambient conditions, while the high temperature specification has increased due to the factors mentioned above, and usually is fixed by the running temperature of the engine.
The engine compartment temperature of today's motor vehicles may reach 120.degree. C. and generally when the vehicle stops after operation and no cooling is exerted from the outside air flow as would be experienced during moving operation, the engine compartment temperatures often may reach 140.degree. C. or even 150.degree. C. Such temperature extremes (high and low), whether endured for a relatively short period of time such as in daily vehicle use, or especially, endured repeatedly for long periods during the vehicle life, put additional stress or demands upon all parts in an engine compartment. Elastomeric compounds for engine compartment use must first function at these temperatures and further must retain a useful life over all or a majority part of the vehicular life which may extend to 10 years or more than 150 thousand miles.
A long life at severe temperature extremes, means generally exposure or aging at high temperature which can be detrimental to materials such as elastomeric parts used in the automobile, and lead part suppliers and vehicle manufacturers to search for materials which will, when fabricated into vibration or noise dampening parts, perform the same function or perhaps even have improved performance at broader temperature ranges, under more severe conditions.
In the past, most vibration dampening devices have been manufactured from compounds based on natural rubber. Natural rubber has been preferred because of its generally high molecular weight, which makes it very resilient. Additionally, even with a low level of reinforcing filler generally used to provide a high resiliency to the part, natural rubber was able to satisfy the severe physical property requirements of the dampening devices. Natural rubber's self-plasticizing charcteristic at compounding temperatures allows a low level of oil or plasticizer in the engine mount compounds while being processable (mixing and molding) by conventional machines. So elastomer compounds targeted to replace natural rubber compounds must generally utilize a combination of relatively low level of reinforcing filler, and low oil or plasticizer, still maintaining adequate compounding parameters, and deliver similar dynamic physical properties as natural rubber, but at a substantially higher service temperature. Most natural rubber compounds have performed relatively well when engine compartment temperatures were in the range of about 80.degree. C. to about 110.degree. C. Their physical properties, after aging or use, either in a vehicle or in testing intended to simulate the environment of an automobile, begin to drop off generally above about 80.degree.-110.degree. C. These properties include, low hysterisis or high resilience featuring a low viscous modulus at vibration frequencies comprised between 10-200 Hz, a low increase of the elastic modulus when the vibration frequence increases, a good creep resistance, a resistance to stiffening at low temperatures, a high tear resistance, and good compression set at elevated temperature.
In the recent past, ethylene, alpha-olefin, non-conjugated diene, elastomeric polymer based compounds have often been suggested as replacements for the majority of the natural rubber based under the hood parts, particularly in the critical applications such as engine mounts because the ethylene, alpha-olefin, non-conjugated diene elastomeric polymers generally maintain physical properties at higher temperature and keep a substantial measure of those properties after long term, high temperature aging.
Additionally, temperature resistance beyond 120.degree. C. can generally only be obtained with a cure system which provides higher crosslink energy than the sulfur cure system. Examples of such better performing cure systems are resin cure, radiation cure or peroxide cure. The most suitable and efficient for an economic and industrial molding cycle is the peroxide cure system which provides temperature resistance up to 160.degree. C. These peroxide cure system are not compatible with the natural rubber since a depolymerization occurs, but are particularly efficient with ethylene, alpha-olefin, non-conjugated diene, elastomeric polymer, because of their chemical structure giving carbon to carbon link after the action of free radical species generated by the decomposition of the peroxide present in the compound as crosslinking agent.
However, most of the currently available ethylene, alpha-olefin, non-conjugated diene, elastomeric polymer compounds contain a diene monomer selected from the group consisting of 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,6 octadiene, 5-methyl-1,4 hexadiene, 3,7-dimethyl-1,6-octadiene, or combinations thereof.
The processability of compounds made from such elastomeric polymers intended for use as engine mounts, may not be optimum, because the molecular weight necessary to provide a similar low hysterisis to natural rubber is generally so high as to potentially interfere with the compounding, especially given the additional limitations placed on these compounds of generally little liquid plasticizer or oil, and relatively little reinforcing filler that can be used to formulate the compound. As these ethylene, alpha-olefin, non-conjugated diene, elastomeric polymer compounds are generally formulated with elastomeric polymer, carbon black, plasticizer, process aids, curatives, and other additives known to those of ordinary skill in the art, with low levels of liquid plasticizers and/or oils the polymer has the double role of being the plasticizing agent during the processing of the compound and providing the best of its elastic properties once cured.
The processability of a given elastomeric polymer or elastomeric polymer compound is of importance in the manufacture of vibration damping devices such as engine mounts for consistency and general quality of production. A material which displays generally a lower viscosity at compounding and molding temperatures without the tendency to prematurely cure or scorch, would be desirable because relatively high viscosity elastomeric polymers cannot get processing assistance (i.e. substantial viscosity lowering) from large amounts of oil or plasticizer.
Improvements in vibration damping part manufacturing economics while maintaining part quality are goals of many part manufacturers. Economies of scale in such a molding operation might include larger presses, and larger molds with more cavities (more parts) to accommodate the larger presses, but such methods are capital intensive and most fabricators might look for other methods to improve economics. Regardless of the methods used, the processability of an elastomeric polymer compound can have a substantial impact on these economies.
A lower compound viscosity could equate to improved ease of compounding and even more mold cavities filled faster. A faster part cure rate could lead to decreased molding cycle times (premature cure or scorch during mold filling is generally to be avoided), another process improvement that could also lead to economies. Both the lower viscosity and faster cure rate could beneficially impact fabrication economics. However, as explained above, a lower compound viscosity for a given elastomeric polymer, will generally be limited by the viscosity of the elastomeric polymer base of the compound during the compounding step, lack of substantial quantity of oil or plasticizer. Further, faster, more complete cures can be had only within very small limits for a given elastomeric polymer, by the type of or amount of curative, and the heat transfer in the mold. For a given elastomeric polymer compound such moves are often restricted by the inherent properties of the elastomeric polymer such as crosslink mechanism and curative type. However, the compounder will often have to compromise between higher levels of curative, which may deliver faster and or more complete cures, and premature scorch. Premature scorch can lead to incomplete mold cavity filling and defects in the part, which is critical for the dynamic characteristics of the part. Additionally, attempting to increase heat transfer can also lead to the same problems and defects.
There is a commercial need, therefore, for an elastomer which, when compounded, can provide vibration dampening parts which have improved resistance to high temperature of service before and after long term aging, maintain low temperature flexibility, have a low hysterisis( high resiliency) at different temperature of service and after aging under a range of frequencies typical of an automotive engine (comprised between 10 to 200 Hz), good resistance to creep, while having improved compound processability as measured by viscosity at high shear and injection temperatures, improved cure rates as measured by time to cure after a mold is filled, and improved or higher cure states.